Cysteine biosynthesis in a fungus

Publisher Website
Google Scholar
Abstract
A step in cysteine biosynthesis was analyzed in several strains of the pathogenic dimorphic fungus, H. capsulatum. Mycelial cells of all strains tested are prototrophic. However, the yeast phase cells of most strains do not grow in the absence of -SH-containing compounds due to the apparent lack of an active form of sulfite reductase, a crucial enzyme in the cysteine biosynthetic pathway. In contrast, the yeast phase cells of 1 strain (Downs) had an active sulfite reductase and can grow in the absence of cysteine if serine is added. A different metabolic block must thus exist in this strain. Sulfite reductase in the yeast form of Downs strain is completely repressed by growth on cysteine while the mycelial form seems to be constitutive. The yeast and mycelial phase extracts were analyzed on polyacrylamide gels. A distinct protein band appeared in extracts prepared from the yeast cells incubated in minimal or serine-containing media, but not in extracts from mycelia or from cysteine-growth yeast cells.